In conventional-scale liquid chromatography, the mobile phase liquid is usually conveyed between components of the chromatography system in tubing constructed from stainless steel, polyether-ether-ketone (PEEK), or Teflon®. Conventional-scale chromatography is practiced with analytical columns having a typical internal diameter in the range of 3.9 to 4.6 millimeters. An industry-standard external diameter for interconnection tubing is nominal 1/16th inch (approximately 0.062″ or 1.59 mm)
The internal diameter of the interconnection tubing will generally vary with the type of application, but diameters ranging from 0.005″ to 0.040″ (0.127 to 1.02 mm) are common. Where liquid-tight connections need to be made between tubes, or between tubes and other structures such as flow cells, columns, or pumps, the sealing interface is often implemented with a compression-type fitting.
Typically, a conventional union is a junction effected between two stainless steel tubes. Stainless steel is a rugged tube material which can be fabricated with a burr-free, square polished end having a substantially right-circular cylindrical form. The union is also constructed from stainless steel and has a compression-type port detail machined into each end.
The cylindrical tube end is inserted into the union until it bottoms in a corresponding flat-bottomed counterbore feature within the port. The insertion depth of the tube is thereby controlled by the depth of the counterbore feature. A deformable ferrule, typically having a conical form, is slid over the tube to a point where it is interposed between the tube and a corresponding conical cavity within the compression port. A compression screw, which is free to rotate and translate over the tube outer diameter, engages a corresponding threaded region of the compression port.
Tightening of the compression screw deforms the ferrule in such a way as to effect a substantially liquid-tight seal between the outer diameter of the tube and at least a portion of the conical cavity of the compression port. The seal thus formed is therefore slightly removed from the actual tube end. The material chosen for the deformable ferrule will vary with the tubing material choice, and with the operating pressure specification, with stainless steel, PEEK, Teflon®, or Vespel® being typical candidate materials. Unions, tees, crosses, and other parts based on compression fitting interfaces are commercially available for industry-standard tube sizes. As is the case with tubing and ferrules, the unions, tees, and other components are manufactured from a variety of materials which address a range of working pressures and chemical compatibilities.
At a conventional liquid chromatography size scale, the tubing and associated parts, such as ferrules and compression screws, are of dimensions which can be readily manipulated by hand in the user's laboratory. Where interconnection tubing engages other tubes or fluid conduits, the alignment requirements are moderate. For example, the interface between two tubes of 0.010″ internal diameter would tolerate misalignment of several thousandths of an inch without resulting in either blockage of the flow path, or serious degradation of the fidelity of chromatographic zones or bands, where the volume of the band is typically several hundred microliters.
Misalignment at the level of several thousandths of an inch can result from the diametral clearance which is required to facilitate assembly of a tube into a compression port, or from concentricity error introduced between the respective internal and external diameters of a tube during the tube manufacturing process. At conventional chromatography scale, maintaining the necessary fluid path alignment is possible with conventional design and machining practices, where attention is paid to the tolerance of the individual components and component clearances.
In recent years, interest has continued to grow in the practice of liquid chromatography at capillary size scales, where the internal diameter of the analytical column may range from 800 microns (micrometers) to 50 microns or less. For a column of 75 micron internal diameter, the volume of an eluting zone or band will typically be less than 100 nanoliters, or several thousand times less than in a conventional-scale chromatographic separation. In order to effect connections between components of a system incorporating a 75 micron diameter column, the connecting tubing will typically be chosen to have an internal diameter of 25 microns (approximately 0.001″) or less.
The number of materials from which practical tubing of 15 to 25 micron internal diameter can be formed or drawn, while maintaining the necessary strength, smoothness, concentricity, solvent resistance, and cost, is relatively few. Fused silica is one such material, and fused silica tubing is commercially available in a variety of internal and external diameters suitable for use in liquid chromatography at the capillary size scale. Commercial fused silica tubing is typically provided with a polyimide buffer coating which provides a degree of mechanical protection to the external surface of the tubing.
While fused silica tubing has many desirable properties, its use in high-pressure liquid chromatography applications is hampered by several difficulties. When cut to length in the field, fused silica tubing is typically scored or nicked with a diamond tool, and then fractured. A poorly-cleaved end usually exhibits one or more projecting shards of fused silica, or projecting flaps of polyimide sheath. Deviation from a right-circular cylindrical form makes it difficult to achieve a joint between tubes, or between tubes and other structures, which does not suffer from poorly swept or “dead” volumes. Dead volumes degrade the fidelity of chromatographic zones which elute past the joint, resulting in broad peaks with correspondingly diminished chromatographic resolution. With time, poorly cleaved fused silica tubing ends may continue to fracture back, releasing fragments of fused silica and polyimide buffer coating, and further widening the gap between the tube end and the adjoining structure. Fused silica or polyimide fragments that become entrained in the liquid flow may cause blockages downstream which render the system inoperable.
Achieving adequate alignment of fluid paths at capillary scale junctions is also problematic. Within the liquid chromatography industry, adaptor sleeves and corresponding ferrules have been developed which are intended to provide a sealing interface between fused silica tubing and conventional compression-type ports. Such sleeves are typically constructed from PEEK, and most commonly are sized with the intent of adapting a fused silica tube of roughly 0.015″ outer diameter to a compression port originally intended for nominal 0.062″ outer diameter tubing.
In practice, the utility of these adaptor sleeves and ferrules is limited. The fused silica tubing itself will often demonstrate reasonably good concentricity between the internal and the external diameters. However, from spool to spool, the external diameter will typically vary over a range as defined by the product specification, but that may be of the order of tens of microns. This tolerance in outside diameter is accommodated by a corresponding clearance dimension designed into the internal diameter of the adaptor sleeve, so that the sleeve can be assembled onto the tube in the field. Correspondingly, there is typically a clearance introduced between the outside diameter of the adaptor sleeve and the internal diameter of the port detail where the sleeve engages it, so that the parts can be assembled in the field. As the adaptor sleeve is subjected to localized compression from the ferrule, the deformation may result in the fused silica tubing being biased toward one side.
Further reduction in the clearance dimensions is impractical, as the assembler is confronted with having to manage the insertion of a component of typically 0.015″ diameter, and to accomplish that insertion without damaging the fragile end of the fused silica, or skiving PEEK or other material from the internal surface of the adaptor, thereby generating fragments which could obstruct the fluid path.
Even in the absence of alignment error resulting from the accumulation of component clearances, there is typically a concentricity error introduced between the inner and outer diameters of the adaptor sleeve itself. As a result, when a given fused silica tube is installed into a compression-type fitting using the adaptor sleeve and ferrule approach, the registration between the lumen or fluid path of the fused silica tubing and the fluid path of the mating part may be so poor as to effectively block the fluid path.
One industry-standard approach to circumventing fluid path blockage arising from errors in alignment of the respective fluid conduits meeting in a device such as a union, is to utilize a relatively larger diameter for the through-hole which penetrates the central web of the union. The diameter of this hole can be chosen to be large enough so that even in the presence of the largest expected accumulated positioning error, continuity of the fluid path can be guaranteed. This prior art approach may avoid obstruction of the fluid path, but tends to produce a junction with excessive band-broadening characteristics when employed at smaller volume scales. The discontinuity in flow path geometry encountered by the chromatographic band as it transits the enlarged central region of the union typically results in poor flushing characteristics with corresponding negative impact on band shape.
Another approach to circumventing fluid path blockage arising from errors in alignment of the respective fluid conduits meeting in a device such as a union is to utilize a single adaptor sleeve which is shared by both tubes where the tubes meet at the union. A union can also have no central web, where the drilled-through hole is dimensioned such that a single adaptor sleeve can pass directly through the device. In this embodiment, the sharing of a single adaptor sleeve by two tubes may reduce some of the accumulated alignment error, but it is still subject to error arising from diametral clearance between the outer diameters of the respective fused silica tubes, and the inner diameter of the adaptor sleeve.
More importantly, there is now no feature of the union which dictates the insertion distance for the respective tubes upon assembly, and the junction region is not accessible to view. Attainment of a liquid-tight seal requires that the junction between the respective fused silica tube ends lies in a region of the adaptor sleeve between the zones of compression produced by the respective ferrules. Accomplishing this on small parts without the assistance of additional fixturing is difficult. This approach is very susceptible to damaging the relatively fragile fused silica tubing ends because assembly of the union in the field, in the absence of visual cues or dedicated fixturing, typically involves a manual process of feeling for contact between the inserted fused silica tube ends, and then holding that contact while tightening is performed on the compression screws.
Moreover, during the tightening of a conventional compression-type fitting, the action of the compression screw tends to impart both a rotational and an axial motion to the adaptor sleeve as the ferrule and sleeve are deformed. This combination of motions tends to drive the fused silica tube end into grinding contact with the opposing tube end, or with the bottom of the port detail when a shared sleeve is not used. This grinding contact can further aggravate the fracturing of the raw fused silica tube end.
Additionally, a compression fitting with adaptor sleeve and ferrule relies on a friction-type connection to retain the fused silica tubing within the sleeve. With conventional stainless steel tubing, an adequately tightened stainless steel ferrule will create a local deformation in the underlying tube such that the tube will not extract from the ferrule in response to normal chromatographic pressures. When a PEEK or other polymeric adaptor sleeve is compressed onto a fused silica tube, though the ferrule may create a deep deformation in the adaptor sleeve, there is substantially no corresponding deformation created by the adaptor sleeve in the fused silica tubing. Therefore, in the presence of high chromatographic pressure, retention of the fused silica tube within the sleeve relies upon friction between the polymeric sleeve and the polyimide buffer coating of the fused silica, or between the polymeric sleeve and a bare fused silica surface. In either case, a common mode of failure is ejection of the fused silica tube from the compression fitting as liquid pressure is increased.